I'm still waiting to hear back from Valve after I sent over some initial info. As soon as I get SDK access I'll be making sure that works, then getting the game out as fast as possible. I don't know how packaging works with Steam w.r.t. Linux (I hear Defender's quest distribueted as deb/rpm in addition to tar.gz , gotta look into the former two), but I know for sure Mac and Windows will be available upon Steam launch. There will likely be a launch sale, though idk how much nor do I know the base price point yet.

Achievements?

Yes.

Controller support?

Controller support works on Mac and Windows! I don't know if it is possible with Linux yet.

Steam keys?

I will do this as soon as I am able to generate keys, though idk if they will be redeemable until the game actually goes live.

Humble Store: Key should show up in your purchase page when I upload them. Buying from here post and pre release will give you keys.

BMTMicro Pre-orderers: I will probably either re-send the fulfillments with Steam keys, or if I can have Steam email you them I will just do that.

Fastspring: I will try to see if I can e-mail them to you via something with Steam. Otherwise, you should have received a Desura key at some point, and if you have the Desura copy you can get a Steam key.